The present invention is directed to apparatus for monitoring an electric power distribution system and in particular to a multiprocessor unit which provides two levels of circuit protection.
In a typical factory power distribution system, high-voltage (i.e. greater than 1,000 volts) power provided by the power company generation station is stepped down to low voltage power using a transformer. The low voltage power is then distributed around the factory to power equipment such as, motors, welding machinery and large computers.
Power distribution systems of this type are typically divided into branches, where each branch supplies power to a portion of the factory. The entire power distribution system is protected by installing low voltage fuses or circuit breakers in each branch so that a fault such as a short circuit in a piece of equipment supplied by one branch will not affect the power distributed to equipment coupled to the remaining branches.
Typically, these low voltage circuit breakers detect more than just large overcurrent conditions caused by short circuit faults. In addition, they detect lower level long-time overcurrent conditions and excessive ground current. The simplest form of circuit breaker is thermally tripped as a result of heating caused by an overcurrent condition. This type of breaker is best for detecting relatively low level overcurrent conditions since it measures the cumulative heating effect of the low-level overcurrent condition over a period of time. A breaker of this type, however, may respond too slowly to provide effective protection against high-current short circuit conditions.
Another type of breaker monitors the level of current being passed through the branch circuit and trips the breaker when the current exceeds a predefined maximum value. Breakers of this type typically include a microcontroller coupled to one or more current sensors. The microcontroller continually monitors the digitized current values using a curve which defines permissible time frames in which both low-level and high-level overcurrent conditions may exist. If an overcurrent condition is maintained for longer than its permissible time frame, the breaker is tripped.
Although this type of breaker provides good protection against both long-time and short-time overcurrent conditions, if it does not calculate RMS current values, it may erroneously trip the circuit when a nonlinear load, such as a welding machine, is coupled to the branch that it is protecting. Non-linear loads tend to produce harmonics in the current waveform. These harmonics tend to distort the current waveform, causing it to exhibit peak values which are augmented at the harmonic frequencies. When the microcontroller, which assumes a sinusoidal current waveform, detects these peaks, it may trip the breaker even though the heating effect of the distorted waveform may not require that the circuit be broken.
Since circuit breakers of the type described above only monitor overcurrent conditions, other types of faults such as over or under voltage conditions and phase imbalances may be missed unless or until they result in an overcurrent fault. Typically, circuit protection for faults of this type requires special purpose line monitoring and relaying equipment, separate from the overcurrent breakers.
Another problem with many existing circuit breakers involves the time required to restore the branch to operation once the breaker has been tripped. For purely transient faults, such as a power surge during an electrical storm, a technician must go onto the factory floor, locate the tripped breakers and reset them. Depending on the experience and knowledge of the technician, this may take a few minutes or a few hours. In this instance, however, the delay may be minimized by using a breaker with an automatic recloser.
Faults caused by the equipment that is powered by the branch may be more difficult to locate. Many circuit breakers provide no information on the conditions present at the time the breaker was tripped. Thus, the technician may need to install power monitors on each piece of equipment to determine the magnitude and duration of the current that caused the fault. Due to the limited information provided by currently available breakers, faults of this type may take several days to locate and correct.
A final problem with existing low-voltage circuit breaker systems concerns the lack of effective backup protection if the breaker should fail to trip. This problem is more of a concern with microcontroller based trip units than with the older thermal trip units. In general, effective backup protection may include a fuse, in series with the branch line, which blows at a short-circuit current slightly higher than the short-circuit current of the breaker. If the microcontroller or any of its associated circuitry fails, a lower-level overcurrent condition may damage the distribution system and/or the equipment being protected before the backup fuse is blown.